Fairings for underwater towing purposes, whereby the coefficient of drag on a towing cable is reduced, are well known. For example, Hale U.S. Pat. No. 3,611,976 issued Oct. 12, 1971 teaches an improved fairing for underwater towing cables which serves to reduce the coefficient of drag on the towing cable to less than 0.1. The fairing has a substantially rigid tail portion and a nose portion which is flexible in the forward direction so that the cable can be reeled--i.e., stowed--on a winch without damage to the fairing and without having to pre-dimension a curve in the nose of the fairing. However, such fairings have been used in diameters ranging from less than 1 cm. to 8 or 10 cm. Recently there has been some considerable interest in the use of fairings to reduce the coefficient of drag on substantially rigid pipes which may be used for such purposes as deep-sea mining of manganese nodules and the like; and as well, on riser pipes in offshore drilling rigs and similar installations.
In such instances, however, the pipe, or pipe bundle, is substantially rigid and assembled in lengths; and it is useful to provide a fairing which can be easily handled and which will accommodate very large pipes in such installations as those noted above. Accordingly, the present invention provides a fairing which basically comprises a pair of shell halves--usually formed from suitable plastics material such as acrylonitrile butadiene styrene or the--where the fairing is quite rigid itself, and comprises a nose portion adapted to accommodate the pipe and a tail portion which provides the streamlining effect. The fairings have a plurality of upstanding rib portions formed therein, at least in the tail portions thereof; and the upstanding rib portions provide a different cross-section taken therethrough than the cross-section taken through the intervening or alternating tail portions between the rib portions, at least in absolute terms of length and breadth. However, notwithstanding the differences in absolute terms of length and breadth of the cross-sections of a fairing according to this invention, taken through any upstanding rib portion and through any alternating tail portion therebetween, the aspect ratios of chord length of the cross-section at least to the rearmost termination of the tapered portion thereof to the breadth of the cross-section, of either cross-section respectively, are substantially similar. Moreover, it is a particular feature of this invention that the aspect ratios of fairings according hereto are very low, generally in the range of 1.8:1 to 4:1, and most usually 2:1 or 2.1:1.
By forming a fairing according to the present invention in two shell halves from a suitable plastics material, the fairing may be assembled by installing a hinge between the shell halves at the parting line thereof along the front of the nose portion of the fairing; and suitable detachable fasteners may be installed in the tail portion. Accordingly, a fairing can be assembled to a pipe either by unhinging the shell halves and fitting the nose portion of the fairing to the pipe, or by fitting the shell halves together and installing a hinge pin and suitable fasteners. Usually, of course, the fairing shell halves are hingably opened with respect to each other and the pipe to which the fairing is to be fitted is accommodated in the nose portion. In any event, the pipes or pipe bundles as referred to hereafter which may be accommodated by fairings contemplated by the present invention may have diameters in the range of from approximately 20 cm. to approximately 1 m. The physical size of such fairings is, therefore, quite large; and in order for the fairings to be easily handled without the necessity for special equipment, they are produced as shell halves of suitable plastic material.
The importance of low aspect ratio fairings now becomes evident, in view of the physical size being handled. For example, an ordinary fairing having an aspect ratio of approximately 5:1 and having a breadth sufficient to accommodate a pipe with a diameter of 0.5 m. would require a length from nose to trailing edge of more than 2.5 m. However, when a fairing is provided having an aspect ratio of length to breadth in the order of 2:1, the restoring forces acting on the tail of the fairing as the relative angle of attack of the fairing to the fluid medium--usually water--changes or shifts, may not be sufficient to cause the fairing to swing sufficiently quickly into position. In such circumstances, strumming or vibration of the fairing and the structure to which it is fitted may occur.
A distinction must be drawn between fairings which will accommodate a shift in direction of relative flow of the faired structure to the fluid medium as opposed to the streamlining fairing of the sort referred to in Schuh U.S. Pat. No. 3,410,096 issued Nov. 12, 1968. That patent teaches an assembly for streamlining a riser pipe of an offshore drilling rig or the like, where the assembly must be critically aligned with respect to the current. The device taught by that patent is said to accommodate a change of direction of flow of current--a change of angle of attack--of plus or minus 30.degree. with respect to the X axis; but where the ellipse ratios become less, the permissive angle of attack may be only 20.degree. or even 10.degree.. The Schuh streamlined riser pipe, therefore, is such that only relatively small changes of current flow can be accommodated; but where the offshore drilling rig is installed in such a place as the mouth of a very large river but is subject to high tide flow, it is important to provide a fairing which has an accommodation of 360.degree. of swing in either direction. Likewise, when towing a pipe for undersea mining purposes, a fairing having cocking capabilities is required.
Yet another application where very large size fairings having short aspect ratios may be required is in the matter of laying fuel pipelines across waterways, especially waterways having high current flow which differs from time-to-time or from top to bottom as the pipeline is being placed. It has been known where the drag on a pipeline produced by current flow of the waterway in which the pipeline is being laid has been sufficiently great as to pull a crane or bulldozer into the water, with the resultant loss of capital equipment as well as of the pipeline itself. Accordingly, another purpose for fairings according to the present invention may be to permit more economical laying of pipelines in waterways, with far less energy expenditure and the commensurate time and labour savings.
In all of the above circumstances, however, where a fairing is used having a short aspect ratio of length to breadth, and where it is important that the restoring force on the fairing be such as to align the fairing with flow as quickly as possible, an improved and preferred embodiment of the present invention provides a fairing having a pair of stabilizer plates secured across the outer rear portions of the ribs of the fairing, on either side thereof. What is provided is, to some extent, analogous to a slotted wing; but the fairing is symmetrical and the restoring force must be capable of operating in either direction, so that differences between the fairing and a slotted wing are substantial.
Indeed, in the preferred embodiment of fairing according to this invention, apertures are formed in trailing fin portions of the fairing which extend behind the intervening tail portions between the rib portions thereof; and which substantially underlie the stabilizer plates secured to the rib portions. These apertures appear to provide for faster restoring forces by accommodating a net flow of fluid medium--usually water--from the one side of the fairing to the other as the fairing swings to re-align itself with the relative stream flow.
When a fairing of the sort provided by this invention is installed on a pipe and immersed in water--either fresh water or sea water--and the interior of the fairing shell is permitted to flood, the fairing has substantially neutral buoyancy. Therefore, no additional bearing weight is provided by the fairing nor does any such additional weight need to be accommodated either from above if the pipe is suspended thereby or from below if the pipe has a net weight downwardly against apparatus installed at its base and on the sea floor. The provision of neutral buoyancy also means, of course, that long pipe lengths can be handled, when faired, without the necessity for special handling tools to accommodate the extra weight of the fairings; because the fairings are, even in free air, relatively quite light.
In order to accommodate pipes of varying diameters, or pipe bundles, collars may be fitted to fairings according to this invention. Such collars are secured to the pipe or pipe bundle, and the swinging motion of the fairing is then accommodated relative to the collars. Indeed, for very long or very large pipes, collars are generally installed either between each adjacent pair of fairings or at least with respect to a small group of fairings. The collars thereby serve several purposes, including the provision of facing surfaces to accommodate the swinging motion of the fairing on the pipe, and the accommodation of different sized pipes in a given fairing or more than one pipe such as a main riser pipe and a choke line, hose bundle and the like. Thus, it may be possible to permit movement of a small-bore exploratory drilling ship from one site to another without recovering and dis-assembling the faired drill casing.